Unless

Unless
 

Are they all corrert?

-He wouldn't understand me, unless he is/was/were in my position.(referring to now)

All can be heard, and can therefore be considered to be correct, but I'll expound on the usage.
The word 'unless' sets a condition, and the auxiliary 'would' is used to make the main verb 'conditional'.

Using 'were' in the secondary clause would be my first choice, because it is in the subjunctive mood.
Most Americans would not naturally choose 'were' (because the subjunctive mood has mostly fallen out of use in American English). They would say 'was'.

Because it doesn't pair well with the conditional form appearing in the main clause, I would not choose 'is'.

You're absolutely right, but I wouldn't say the subjunctive in English has fallen out of favor. Most properly educated people would use "were" in that context. Sadly, the knowledge of the English language among the average US citizen has dropped drastically since I've been alive.

I strongly suggest using were.
It sounds very sloppy otherwise. Just because was is often used in this case, doesn't mean it's right.

Thank you~